Process for separating dimethyl naphthalenes comprising 2,6-dimethyl naphthalene as main component

ABSTRACT

A process for separating dimethyl naphthalenes the main component of which is 2,6-dimethyl naphthalene in the form of their complexes with m-nitrobenzoic acid, which comprises contacting a dimethyl naphthalene isomer mixture comprising at least 2,6-dimethyl naphthalene or a hydrocarbon mixture containing such dimethyl naphthalene isomer mixture with mnitrobenzoic acid, to thereby form a mixture of complexes of the dimethyl naphthalenes with m-nitrobenzoic acid the main component of which is a complex of 2,6-dimethyl naphthalene with mnitrobenzoic acid, and separating the complexes in the solid state from the reaction mixture.

United States Patent [191 Nagahama et al.

3,725,490 Apr. 3, 1973 PROCESS FOR SEPARATING DIMETHYL NAPHTHALENESCOMPRISING 2,6-DIMETHYL NAPHTHALENE AS MAIN COMPONENT Inventors: ShizuoNagahama; Keizo Shimada;

Seizi Kuroziimi, all of Tokyo, Japan Assignee:

Filed:

Teijin Limited, Osaka, Japan Mar. 30, 1971 Appl. No.: 129,621

Foreign Application Priority Data Dec. 22, 1970 Japan ..45/1 16410 Mar.30, 1970 Japan Mar. 31, 1970 Japan Mar. 31, 1970 Japan ..45/27302 Apr.1, 1970 Japan ..45/27697 Oct. 3, 1970 Japan ....45/96047 Nov. 12, 1970Japan ..45/99074 US. Cl. ..260/674 N Int. Cl..- ..C07c 7/02 Field ofSearch ..260/674 N [56] References Cited UNITED STATES PATENTS 3,665,0445/1972 Scott ..260/674 2,914,581 11/1959 Christensen et al. ....260/6742,941,017 6/1960 Veatch et al. ....260/674 2,941,019 6/1960 Foreman ctal. ..260/674 Primary Examiner-Delbert E. Gantz Assistant ExaminerC. E.Spresser Att0rneySherman & Shalloway [57] ABSTRACT 10 Claims, NoDrawings dimethyl PROCESS FOR SEPARATING DIMETI-IYL NAPI-ITIIALENESCOMPRISING 2,6-DIMETIIYL NAPIITHALENE AS MAIN COMPONENT bon mixturecontaining such isomeric mixture with mnitrobenzoic acid.

2,6-DMN can be converted to naphthalene-2,6- dicarboxylic acid byoxidation, and the resulting naphthalene-2,6-dicarboxylic acid is usefulas the starting material for the preparation of polyesters andplasticizers.

Also other DMN isomers can be converted to naphthalene-2,6-dicarboxylicacid by oxidizing them and then subjecting the oxidized products to thesocalled Henkel rearrangement reaction.

Accordingly, various proposals have been made to separate DMN mixturescomprising 2,6-DMN in the highly concentrated state from a mixture ofDMN isomers comprising 2,6-DMN or a hydrocarbon mixture containing suchmixture of DMN isomers.

For instance, a DMN mixture comprising 2,6-DMN and 2,7-DMN as maincomponents can be obtained by cooling a DMN-containing fractionconcentrated and extracted from a petroleum or coal tar startingmaterial by a suitable method. The resulting DMN mixture isrecrystallized from a suitable solvent. This recrystallizing method isdisclosed in, for instance, the U.S. Pat. No. 3,249,644. In U.S. Pat.No. 3,485,885 and U.S. Pat. No. 3,400,548 a partially melting process isproposed comprising melt-separating low melting point components of theabove DMN mixture or a low melting point eutectic mixture, from such DMNmixture derived from a petroleum or coal tar starting material. Byadopting these methods or a combination of these methods 2,6-DM N can'beseparated but the yield of 2,6-DMN is very low.

2,6-DMN and 2,7-DMN form a eutectic mixture at a molar ratio of4l.5':58.5, and 2,6-DMN and 2,3-DMN form a eutectic mixture at a molarratio of 4752525 Accordingly, if the starting DMN mixture contains 2,7DMN or 2,3-DMN as well as 2,6-DMN, a eutectic mixture of 2,6-DMN with2,7- or 2,3-DMN is formed and it is impossible to separate 2,6-DMN fromthe starting mixture in which such eutectic mixture has been formed, bythe above recrystallizing or partially melting method. In an ordinarystarting mixture, for instance, 2,6-DMN is contained at a content ofabout ll --12 percent by weight, and 2,7-DMN is contained at a similarcontent. In such starting mixture, about 4/6 mole of 2,6-DMN per mole of2,7-DMN forms a eutectic mixture with 2,7-DMN present in the startingmixture. Accordingly, the 2,6-DMN which can be actually separated is atmost 30 percent or less of the entire 2,6- DMN present in the startingmixture, and although it is possible to increase the content of 2,6-DMNby fractional rectification up to about 30 percent, since the boilingpoint of 2,7-DMN is very close to the boiling point of 2,6-DMN it isimpossible to change the proportions of 2,6-DMN and 2,7-DMN greatly.Therefore, the yield of pure 2,6-DMN cannot beincreased by the aboverecrystallizingor partially melting method.

Accordingly, the primary object of this invention is.

to provide a process by which a DMN mixture containing 2,6-DMN at a veryhigh concentration can be separated in high yield from a mixture ofisomers of DMN.

Another object of this invention is toprovide a 0 process by which a DMNmixture containing 2,6-DMN at a very high concentration can be separatedin high yield from a hydrocarbon mixture comprising 2,6- DMN and one ormore other DMN isomers.

Still another object of this invention is to provide a process for thepreparation of a complex of 2,6-DMN with m-nitrobenzoic acid and aprocess for the purification of such complex;

' Still another object of this invention is to provide a novel processfor decomposing a complex of 2,6-DMN with m-nitrobenzoic acid.

Other objects and advantages of this invention will be apparent from thedescription given hereinbelow.

In accordance with this invention, a process is provided for separatingDMN the main component of which is 2,6-DMN in the form of theircomplexes with m-nitrobenzoic acid, which comprises contacting a DMNisomer mixture comprising at least 2,6-DMN or a hydrocarbon mixturecontaining such DMN isomer mixture with m-nit'robenzoic acid, to therebyform a mixture of complexes of the DMN with m-nitrobenzoic acid the maincomponent of which is a complex of 2,6- DMN with m-nitrobenzoic acid,and separating the complexes in the solid state from the reactionmixture.

An idea was conceived of using a compound capable of forming a complexwith DMN in separating a mixture of isomers of DMN from a hydrocarbonmixture containing such DMN isomer mixture, and various compounds wereexamined with respect to the capability of forming a complex with DMN.Compounds such as picric acid, trinitrotoluene and trinitrobenzene mayform complexes with DMN, but it was found that no selectivity amongisomers of DMN was observed in the complex-forming capability of thesecompounds. It was also found that o-cyanobenzoic acid, 'mcyanobenzoicacid, p-cyanobenzoicacid, o-nitrobenzoic acid, p-nitrobenzoic acid andmethyl esters of these acids do not form a solid separable complex withany of the DMN isomers. Surprisingly, it has now been found thatm-nitrobenzoic acid which has a structure similar to that of theabove-mentioned compounds has a capability of forming complexesselectively with 2,6-DMN and 2,7-DMN among DMN isomers, especially with2,6-DMN, and that by utilizing such peculiar property of m-nitrobenzoicacid it is possible to selectively separate 2,6- and 2,7-DMN, especially2,6-DMN from a mixture of DMN isomers or a hydrocarbon mixturecontaining such isomeric mixture.

In general, mixtures of DMN isomers and hydrocarbon mixtures comprisingsuch isomeric mixture are liquid at room temperature. When thesemixtures are allowed to contact m-nitrobenzoic acid in accordance withthis invention, m-nitrobenzoic acid forms solid complexes selectivelywith 2,6- and 2,7-DMN, especially with 2,6-DMN. The resulting solidcomplex can be separated easily from the liquid mixture by a knownliquid-solid separation technique such as filtration and centrifugalseparation.

This invention will now be detailed.

STARTING MATERIAL In this invention any DMN isomer mixture comprising atleast 2,6-DMN as the starting material may be used. In this invention,hydrocarbon mixtures containing a DMN isomer mixture comprising at least2,6- DMN may also be used. For instance, a fraction of 250-270C.obtained from coal tar comprises about 7 10 percent by weight of2,6-DMN, about 7 10 percent by weight of 2,7-DMN, 45 55 percent byweight of other DMN isomers, and various aromatic compounds such asbiphenyl, monomethyl naphthalene, and monoethyl naphthalene. Further, anaromatic fraction of 250 270C. extracted from the reaction productobtained by the thermal cracking of a petroleum fraction of 200 300C.comprises about 10 13 percent of 2,6-DMN, about 10 13 percent of 2,7-DMN, about 50 60 percent of other DMN isomers, and about 10 20 percentof other aromatic compounds. In addition, various mixtures of DMNisomers comprising 2,6-DMN are contained in bottom oils of petroleumreformers.

In this invention, any aliphatic, alicyclic or aromatic hydrocarbonmixture containing a mixture of DMN isomers comprising at least 2,6-DMN,such as those mentioned above may be used. However, when the startingmaterial mixture contains naphthalene derivatives substituted by 3 ormore methyl groups, such as trimethyl naphthalenes and tetramethylnaphthalenes, since these naphthalene derivatives also form solidcomplexes with m-nitrobenzoic acid, it is necessary to separate thesecomplexes of such naphthalene derivativesafter the treatment withm-nitrobenzoic acid. Although such separation is possible, the amount ofmnitrobenzoic acid used must be increased by the amount used forformation of complexes with such naphthalene derivatives and anadditional step of separating these unintended complexes must beconducted. Therefore, it is preferred that the starting material mixturedoes not contain such naphthalene derivatives.

Even if the starting material to be used in this invention containssmall amounts, for instance, less than about 10 percent, of anitrogen-containing compound such as quinoline and indol and asulfur-containing compound such as thionaphthene, no particulardisadvantage is brought about.

In this invention it is preferable to use as the starting materialmixture a hydrocarbon fraction boiling below 275C., especially at 250270C., because such fraction is free of naphthalene derivativescontaining 3 or more methyl groups and it contains a mixture of DMNisomers at a relatively high ratio. However, this invention is not atall limited to the use of such hydrocarbon fraction.

FORMATION OF COMPLEX AND SEPARATION THEREOF In this invention, thecomplex-forming reaction is allowed to proceed by contacting theabove-mentioned mixture of DMN isomers comprising at least 2.6-DMN orhydrocarbon mixture containing such isomeric mixture, in the liquidstate with m-nitrobenzoic acid.

The critical condition for advancing the complexforming reaction is thatthe DMN isomer mixture or the hydrocarbon mixture containing such DMNisomer mixture is contacted with m-nitrobenzoic acid in a manner suchthat the isomeric mixture or hydrocarbon h mixture will be kept in theliquid state during the contact. Accordingly, when the starting mixtureis solid at point. During contact, m-nitrobenzoic acid may be.

either in the solid state or in the liquid state. However, in order tocomplete the complex-forming reaction in a short period of time, it ispreferable to add mnitrobenzoic acid to the starting isomeric mixture orhydrocarbon mixture and mix them in the liquid state under heat.Preferable heating temperatures range from 60l 50C. When the reaction iscarried out while maintaining both in the dissolved state as describedabove, the resulting liquid reaction mixture is cooled and precipitatedsolids are separated from the reaction mixture. The precipitated solidis composed of complexes of m-nitrobenzoic acid with isomers of DMN, themain component of which is the complex of mnitrobenzoic acid with2,6-DMN. It is sufficient to conduct the above cooling only so thatsolid precipitates may be formed. In general, the higher the temperaturereducing degree during this cooling step, the more selectivelyprecipitated is the complex of mnitrobenzoic acid with 2,6-DMN. On theother hand, when the cooling is effected to too low a temperature atendency appears that the content of the 2,6-DMN complex is lowered inthe precipitated solids and amounts of complexes of other isomers suchas 2,7 DMN, l,6-DMN, 1,7-DMN and 2,3-DMN are increased. In accordancewith the process of this invention, the complex of 2,6-DMN is generallyprecipitated predominately at first, followed by formation of thecomplex of 2,7-DMN. 1

It is preferred that the amount of m-nitrobe'nzoic acid to be used forformation of such complex is 0.1 5 moles, especially 1 4 moles per moleof 2,6-DMN contained in the starting material, and is less than 2 moles,especially less than 1.5 moles, per mole of the total of DMN isomerscontained in the starting material. When m-nitrobenzoic acid is used inan amount within the above range, a complex of m-nitrobenzoic acid with2,6-DMN is formed at a high catching ratio and a high selectivity. Inthe present specification-the term catching ratio means a-ratio of the2,6-DMN separated in the form of the complex to the total of 2,6- DMNcontained in the starting mixture of DMN isomers.

When the above-mentioned complex-forming reaction is carried out in thepresence of a monocyclic aromatic hydrocarbon of six to nine carbonatoms in an amount of 0.1 1 part by weight, especially 0.2 0.5 part byweight, based on the total of the DMN isomers in the starting materialmixture, the complex of mnitrobenzoic acid with 2,6-DMN can be formed ata higher catching ratio and a higher selectivity.

The 2,6-DMN catching ratio varies depending on the amount ofm-nitrobenzoic acid used, and if it is used in a large amount, 2,6-DMNcan be caught almost completely. However, in this case, the purity of2,6- DMN obtained by decomposition of the resulting complex is low. Whenm-nitrobenzoic acid is used in a small amount, the catching ratio islowered, but the purity of the final 2,6-DMN product is increased. Inthis invention, 2,6-DMN can be separated in the form of the complex withm-nitrobenzoic acid at an extremely high catching ratio and a highselectivity by using mnitrobenzoic acid in an amount within the abovementioned range and, if desired, conducting the complexforming reactionin the presence of the above-mentioned monocyclic aromatic hydrocarbon.

' Thus, in accordance with the process'of this invention, when a DMNmixture containing, for instance, 1 l percent of 2,6-DMN and almost thesame amount of 2,7-DMN is mixed with 12 mole percent of mnitrobenzoicacid, by one operation a complex mixture comprising 60 70 percent of the2,6-DMN complex can be obtained. In this case the filtrate containsabout 6 8 percent of 2,6-DMN, and the 2,6-DMN catching ratio is 40percent. When the same DMN isomer mixture is contacted with 50 molepercent of mnitrobenzoic acid, a complex mixture containing 45 percentof the 2,6-DMN complex is obtained and the filtrate contains about 0.5 2percent of 2,6-DMN. In this case,the 2,6-DMN catching ratio is as highas 90v -.95 percent. When a 2,6-DMN of a higher purity is wanted, a DMNisomer mixture comprising 35 70 percent of 2,6-DMN which is obtained bydecomposing the complex mixture formed above, is subjected to the abovetreatment again. For instance, when a DMN isomer mixture containing 2,6-DMN at a content of 50 percent is treated with m-nitrobenzoic acid in amanner as described above, a complex mixture comprising 75 85 percent ofthe 2,6-DMN complex can be obtained. When this procedure is furtherrepeated twice, a complex mixture comprising more than 98 percent of the2,6-DMN complex can be obtained.

Further, vit was found that complexes of mnitrobenzoic acid with DMNisomers other than 2,6- DMN are easily converted to the 2,6-.DMN complexin the presence of 2,6-DMN. Accordingly, when complexes formed from thestarting DMN-isomer mixture and m-nitrobenzoic acid are not decomposedbut are treated together with a fresh starting mixture, the content of2,6-DMN in the resulting mixture can be gradually increased. Forinstance, if a DMN-m nitrobenzoic acid complex mixture comprising 40percent of the 2,6-DMN complex is mixed with a starting materialcomprising 11 percent of 2,6-DMN and almost the same amount of 2,7-DMNand the resulting mixture is heated, melted and cooled, a complexmixture comprising about 60 70 percent of the 2,6-DMN complex can beobtained.

In accordance with this invention, the above mixture of complexes ofm-nitrobenzoic acid with DMN isomers, the main component of which is the2,6-DMN complex, is precipitated in the form of stable solid crystalspreferably by cooling it appropriately. The cooling temperature isdetermined by the desired amount of the precipitated crystals. Ingeneral, it is preferred that the precipitation is performed at atemperature lower by at least 5C. than the temperature adopted at thecomplex-forming reaction and within a range of room temperature to 70C.Of course, no particular disadvantage is brought about if theprecipitation of solid crystals is effected at a temperature a littlelower or higher than the above range.

Precipitated crystals can be separated from the liquid xylene, trimethylbenzene, ethyl benzene, methyl ethyl benzene and cumene may be cited. Asthe ether, those preferably used are diethyl ether, diisopropyl ether,tetrahydrofuran and dioxane. As the alcohol, methanol, ethanol andisopropyl alcohol are preferred.

Acetone and methyl ethyl ketone are especially preferred as the ketone.Ethyl acetate is most preferred as the ester. It is also possible to usea mixed solvent comprising two or more of the above solvents. In casethe purification by recrystallization is conducted, the complex of asuitable amount is dissolved under suitable heating conditions in asolvent such as recited above by a known method, then the solution iscooled to a suitable temperature, and the precipitated crystals areseparated therefrom by a known method.

DECOMPOSITION OF COMPLEX In order to decompose a mixture of complexes ofmnitrobenzoic acid with DMN isomers, which has thus obtained inaccordance with this invention and the main component of which is thecomplex of mnitrobenzoic acid with 2,6-DMN, to a mixture of DMN isomersand m-nitrobenzoic acid, it is preferable to use in principal thefollowing four methods.

A. A method comprising heating the mixture of complexes at a temperaturenot lower than 50C. .but preferably up to the decomposition of benzoicacid (which will be referred to as decomposition method A hereinbelow B.A method comprising contacting the mixture of complexes with a solventwhich hardly dissolves m-nitrobenzoic acid but dissolves DMN well (whichwill bereferred to as decomposition method B hereinbelow). t

C. A method comprising contacting the mixture of complexes with asolvent which hardly dissolves DMN but dissolves m-nitrobenzoic acidwell (which will be referred to as decomposition method C hereinbelow).

D. A method comprising contacting the mixture of complexescoincidentally or alternately with solvents used in above methods B andC which are immiscible with each other (which will be referred to asdecomposition method D hereinbelow).

A. DECOMPOSITION METHOD A As the decomposition method belonging to .thisgroup, a method may be cited comprising heating the mixture of complexesat a temperature not lower than 50C. preferably in an atmosphere of aninert gas. Although the decomposition may be accomplished by heating thecomplex mixture at a temperature not lower than 50C. in the air, it ispreferable to conduct In conducting the heating of the complexmixturepreferably in an inert gas atmosphere, it is possible to recoverthe DMN mixture composed mainly of 2,6- DMN coincidentally withdecomposition of the complex mixture according to the distillation orvolatilization technique by heating the complex mixture at a temperatureexceeding the distilling or volatilizing temperature.

Further, in conducting decomposition of the complex according to themethod A, it is possible to heat the complex mixture in a solvent to beused in the decomposition method B, C or D, which will be detailedbelow, at a temperature exceeding 50C.

The upper limit of the heating temperature is the decompositiontemperature of 2,6-DMN, and any temperature may be adopted as long as itis not lower than 50C. but up to the decomposition temperature of 2,6-DMN. However, in view of the recovery and reuse of rn-nitrobenzoic acid,it is preferable to conduct the heating at a temperature lower than thedecomposition temperature of m-nitrobenzoic acid. Especially preferableheating temperatures are within a range'of 80 to 150C.

Still further, it is possible to conduct the above heating fordecomposition while keeping the complex mix ture in the state dissolvedin a recrystallization solvent. In this case, it is preferred that a DMNmixture composed mainly of 2,6-DMN is recovered by blowing, forinstance, steam into the solvent solution of the complex mixture anddistilling off the DMN mixture composed mainly of 2,6-DMN together withthe vapor of the recrystallization solvent. In general, m-nitrobenzoicacid is soluble in such recrystallization solvent. Accordingly, if thedecomposition is accomplished by the above-mentioned steam-distillationmethod, mnitrobenzoic acid is left in the dissolved or partiallysuspended state, depending on the temperature, in the water which is notdistilled.

B, DECOMPOSITION METHOD B The decomposition method B comprisescontacting the complex mixture closely with a solvent which hardlydissolves m-nitrobenzoic acid but dissolves DMN well.

As such solvent there may be preferably used aliphatic or alicyclichydrocarbons of three to nine carbon atoms, especially three to sevencarbon atoms, saturated ones being particularly preferred, such aspropane, butane, petroleum ether, pentane, hexane, heptane, octane,ligroin, cyclopentane and cyclohexane. Hydrocarbons of more carbon atomsare not preferred because the solubility of m-nitrobenzoic acid in thesehydrocarbons tends to increase. The amount of the solvent used is notcritical, but as the amount of mnitrobenzoic acid dissolved increases inproportion to the amount of the solvent used, it is preferable to usethe solvent in a smaller amount, for instance, 0.5 10 parts by weightper part by weight of the complex mixture to be decomposed.

The m-nitrobenzoic acid dissolved in the solvent may be removed byextraction with hot water after the decomposition treatment.

In conducting the decomposition by contacting the complex mixture withsuch solvent, it is preferable to heat the system at a temperature notlower than 50C. but up to the decomposition temperature of 2,6-DMN,

, especially up to the decomposition temperature of mnitrobenzoic acid,particularly preferred temperatures being in the range of to C., asdescribed with respect to the decomposition method A.

C. DECOMPOSITION METHOD C According to this decomposition method C, thecomwater or a water-containing lower aliphatic alcohol,

especially water, is preferred. The use of a lower aliphatic alcohol,however, is advantageous in that the purification of the resulting DMNoccurs mixture composed mainly of 2,6-DMN coincidentally with thedecomposition of the complex mixture.

As the alkali used for the alkaline aqueous solution,

water-soluble hydroxides, oxides and carbonates of alkali metals andalkaline earth metals; aqueous am monia; and water-soluble amines may beused. The use of aqueous ammonia is especially preferred in actualoperation.

In this method C, the temperature conditions as described with respectto the decomposition method B may be similarly adopted. However, in thismethod C, especially if an alkaline aqueous solution or a loweraliphatic alcohol is used, the decomposition of the complex can beaccomplished rather smoothly at a temperature lower than 50C., forinstance, at room temperature. I

The amount of the solvent used is not particularly critical in the caseof water or an alkaline aqueous solution. When a lower aliphatic alcoholis used, it is preferable to use it in an amount of 2 10 parts byweight, especially 3 6 parts by weight, per part by weight of thecomplex mixture.

As the lower aliphatic alcohol, methanol and ethanol are preferablyused.

D. DECOMPOSITION METHOD D In this decomposition method D, at least onesolvent described with respect to the method B and at least one solventdescribed with respect to the method C, which are immiscible with eachother, are used, and the complex mixture is contacted coincidentally oralternately with such solvents.

In this decomposition method D, the use of a combination of v i. atleast one saturated hydrocarbon of three to nine carbon atoms,especially three to seven carbon atoms, such as described with respectto the decomposition method B, and

ii. at least one solvent selected from water and alkaline aqueoussolution is especially preferred.

As described above, the amount of the solvent (ii) used is notparticularly critical, while it ispreferable to use the solvent (i) inan amount of 0.1 parts by weight, especially 0.5 3 parts by weight, perpart by 4 weight of the complex mixture.

When the decomposition of the complex mixture is carried out inaccordance with this method D, each of the solvents (i) and (ii) ischarged in a decomposition vessel, a suitable amount of the complexmixture is added thereto, and the mixture is well blended and agitated.In this case, when water is used as the solvent (ii), it is preferableto heat the system at a temperature of 80 150C. When an alkaline aqueoussolution is used as the solvent (ii), it is unnecessary to effectheatmg.

It is preferable to conduct the agitation until both of the solvents (i)and (ii) become transparent. By such procedure, the complex mixture isdecomposed to a DMN mixture composed mainly of 2,6-DMN which issubstantially dissolved in the layer of the saturated hydrocarbon (i)and m-nitrobenzoic acid which is substantially dissolved in the layer ofthe solvent (ii). Then, both layers are separated from each other, andwhen the aqueous layer is cooled, m-nitrobenzoic acid is precipitated.In the case of the use of an alkaline aqueous solution, rn-nitrobenzoicacid is precipitated by addition of a mineral acid. A DMN mixturecomposed 1 mainly of 2,6-DMN can be separated and recovered bydistilling off a part of the solvent (i) from the solvent layer and thencooling it, or by distilling off substantially the whole of the solvent(i).

As the decomposition method D a method may be adapted comprisingagitating and mixing the complex mixture in the solvent (i) preferablyunder heating, adding thereto the solvent (ii), and agitating themixture, if required, under heating. In this method, the order ofaddition of the solvents (i) and (ii) may be reversed.

I The so recovered DMN mixture comprising 2,6- DMN as the main componentmay be furth'erpurified by the above-mentioned recrystallizing or knownpurification method, if desired.

naphthalenes and 15.3 percent of other hydrocarbons, were mixed with 300g of m-nitrobenzoic acid, and the mixture was heated at 110C. to form asolution. Then, it was cooled to room temperature. The precipitatedyellow solids were subjected to'a centrifuge rotated at 3300 r.p.m. for20 minutes to obtain 400 g of a crude complex product comprising 55.3percent of the 2,6- DMN complex and 20.1 percent of 2,7-DMN complex.

EXAMPLE [-2 Eight hundred grams of the same hydrocarbon mixturecontaining DMN, as used in Example [-1 were mixed with 400 g ofm-nitrobenzoic acid, and the mixture was heated at 100C. to form asolution. Then, it

was cooled to room temperature and the precipitated solids wereseparated by filtration and subjected to a centrifuge rotated at 2400r.p.m. for 20 minutes to obtain 556 g ofa crude complex productcomprising 50.6 percent of the 2,6-DMN complex and 20.1 percent of the2,7-DMN complex. In the mother liquor left after the separation of thecomplex, there were 0.8 percent of 2,6-DMN and 8.9 percent of 2,7-DMN.

EXAMPLE I-3 EXAMPLE I-4 To 40.7 g of a DMN mixture comprising 25.0percent each of 2,6-DMN', 2,7-DMN, 1,6-DMN and 1,5 DMN, 10.7 g ofm-nitrobenzoic acid were added, and the mixture was heated to form asolution. Then, it was cooled to room temperature (18C.),' and theprecipitated solids were separated by filtration and TABLE I-l AmountDMN composition in complex (percent) Amount Yield of 01 DMN Catching Runof m-NBA complex in complex ratio Number (g.) (g.) (g.) 2,6-DMN 2,7DMNLG-DMN 1,5-DMN (percent) No'rE.m-N BA abbreviation of ni-nitrobmizoicacids This invention will now be described by referring to I examples,but these examples are given for illustration of this invention and thisinvention is not at all limited by these examples.

EXAMPLE I-l Six hundred grams of a hydrocarbon mixture containing DMN,obtained by concentrating a petroleum fraction, the hydrocarbon mixturecomprising 12.0 percent of 2,6-DMN, 12.0 percent of 2,7-DMN, 53.8percent of other DMN isomers, 6.9 percent of ethyl washed with 50 ml ofpetroleum ether to obtain 15.6 g of a crude complex product.

To 50.0 g of a starting DMN mixture of the composition indicated inTable [-2, 25.0 g of m-nitrobenzoic acid were added and the mixture wasmaintained at 80C. for 10 minutes to form a homogeneous solution. Thenit was allowed to cool. The precipitated crystals were separated byfiltration, washed with petroleum ether and dried to obtain 34.0 g of acrude complex product. Then it was treated with a percent aqueoussolution of caustic soda to obtain 10.7 g of a mixture of DMN isomers.The results of the analysis are shown in Table [-2. The 2,6-DMN catchingratio was 92 percent.

TABLE [-2 Composition Composition of DMN of starting Composition mixtureseparated mixture of filtrate from complex by wt.) by wt.) by wt.)2,6-DMN 10.8 1.0 45.7 2,7-DMN 12.1 8.4 22.7 Other DMN isomers 54.8 60.424.4 Ethyl naphthalenes 6.9 8.3 2.3 Other hydrocarbons 11.9 20.7 2.8

EXAMPLE l-6 To 34.0 g of a crude complex product of the same compositionas that of the complex obtained in Example I-5, 50.0 g of a starting DMNmixture of the same composition as that of the starting mixture used inExample I-5 were added and the mixture was heated at 90C. to form asolution. Then it was allowed to cool to precipitate 33.8 g of acomplex. The results of the analysis of the DMN composition of thecomplex are shown TABLE [-4 Composition of hydrocarbons HydrocarbonComposition contained in composition in of starting washing liquorcomplex before mixture after washing decomposition by weight) (k byweight) by weight) 2,6-DMN 45.7 11.8 78.8 2.7-DMN 22.7 31.0 13.4 OtherDMN isomers 24.4 47.3 6.7 Ethyl naphthalenes 2.3 4.3 0.23 Otherhydrocarbons 2.8 5.4 0.94

EXAMPLES I-8 and [-12 To a mixture of an amount indicated in Table l-5of toluene and 10.0 g of a DMN mixture comprising 11.4

percent by weight of 2,6-DMN, 12.9 percent by weight of 2,7-DMN, 53.3percent by weight of other DMN isomers, 7.3 percent by weight of ethylnaphthalenes and 10.6 percent by weight of other hydrocarbons, 3.5

g of m-nitrobenzoic acid were added and the mixturewas heated for 10minutes maintained at 80C. to form a homogeneous solution. Then it wasallowed to cool and precipitated complex crystals were washed withpetroleum ether. In order to determine the 2,6-DMN purity in theresulting complex, it was shaken with a 5 ing DMN mixture are shown inTable [-5. As a comparison, the above procedure was repeated without useof toluene, and the results of this comparative run are To 10.7 g of theDMN mixture obtained in Example 55 L5, 10.5 g of m-nitrobenzoic acidwere added and the mixture was dissolved in ether. The ether was thenevaporated to precipitate a complex product, which was then washed withpetroleum ether. The yield was .in Table l-3. also shown in Table I-5.

TABLE I5 Hydrocarbon composition in complex (percent by weight) AmountYield 2,6-DMN 0i Ethyl Other catching Example toluene complex Otheruaphthahydroratio Number: (g. (g.) 2,6-DMN 2,7-DMN DMNs lencs carbons(percent) 1-8 10. 0 4. 23 51.1 10. 0 1s. 5 1. 3 a. 2 72 1-o 5. 0 4.4151.1 19.4 15. 5 1. s 1. n 75 I10 s. 0 4. s3 55. 5 20. 3 18.9 1. 5 2. 375 I-11 2. 0 4. s5 54. 0 21. 4 20.0 1. 7 2. 5 73 I-12 1. 0 4. ss 52. 321. s 21. a 2. 0 2. s 71 Comparison 0 5. 07 50. 2 21. 1 22. 6 2.0 2. 771 TABLE [-3 EXAMPLES I-13 TO I-l 5 Hydrocarbon Hydrocarbon One hundredgrams (100.0 g) of the DMN containcomposition composition of inghydrocarbon mixture used in Example [-1 were Starting complex resultingcomplex (96 by weight) (96 by weight) contacted with 50.0 g ofm-nitrobenzolc acid at a tem- 5;; 3 perature indicated in Table I-6, andthe mixture was J- 2Z7 cooled to 30C. The precipitated solids wereseparated Other DMN isomers 24.4 16.7 b H h d l f Ethyl naphthalenes 2314 y 1 tration, was c with m o petroleum ether and Other hydrocarbons2.8 1.9 dried to obtain a complex product. Then, it was decom- EXAMPLEL7 posed in the same manner as in Example [-4 to obtain a DMN mixture.The result of the analysis of the DMN mixture are shown in Table I-6.

posed in the same manner as in Example [-5 to obtain 5.06 g of a DMNmixture. The results of the analysis of the mixtureand of the petroleumether washing liquor after washing are shown in Table 14. In thisExample,

60 15.3 g. The so obtained complex product was decomthe 2,6-DMN catchingratio was 78.5.

TABLE I-6 Composition of resulting DMN mixture (percent by weight)Contact. Amount 2,6-DMN tnmpora- Ylclrl of of DMNs Ethyl Other catchingturv complex in com Other nnpl1tlu1- hydroratio (2.) 2.) plux (g.)2,li-l)1\1N LT-DMN DMN'S lnnos carbons (percent) Example Numlwr:

l 13".. .22... 50. (i 20.7 .3 1.0 3.5 $13 |--l'1, (ill 20.) 51.11 20.121.5 1.0 2.! X1! 1 1.5.. an m. 1 5x. 0 10.3 i 2. 4 4. 2

EXAMPLES I-16 AND 147 One hundred grams (100.0 g) of the DMN-containinghydrocarbon mixture used in Example [-1 were contacted at 100C. with50.0 g of m -nitrobenzoic acid gas chromatography. The remainder of thecomplex product was recrystallized again from toluene to yield 2.13 g ofa refined complex product, which was analyzed in the same manner asabove. The results are shown in Table 11-1. and the mixture was cooledto a temperature indicated in Table l-7. The precipitated solids wereseparated by T E [1-1 filtration, washed with an amount indicated inTable l-7 Composition of Composition of of petroleum ether and dried toobtain a complex compoismon complex product complex product product. Itwas then decomposed in the same manner of 818111118 f ig E COI'I'IP 6Xrecrys 1Z3. lOll recrys 123 1011 as in Example l-4. The results areshown in Table [-7 by weight) by weight) by weight) together with theresults of Example H 3. 2,6-DMN 45.7 84.2 97.7

TABLE I-T Composition 01 resulting DUN mixture (percent by weight.)(Jrystal- Amount. Amount lizntion Yield of of l)1\/lN'S of pvtroln-Ethyl ()thcr (.alohing Example 1.11m nm'acomplex in 00111- 11111 other11=1 1h1.h:1- hydroratio Numhvr turn (1.) (gm) plex 0:.) (1111.) ZJHHIN1,7-DMN l).\l.\"s lums 1*;11110115 (pvrcunt) 30 00. 5 22. 2 70 F10. (i'20. 7 23. 3 1.0 3. F1 03 no 56. 2 17.11 70 511.3 1&3 21. 2 1.11 21'-14... at) an. o 12. 4 so as. a 1n. 7 15. 2 1. 3 1. s 67 EXAMPLE II-l2,7-DMN 22.7 13.1 2.3 Other DMNS 24.4 2.0 0.0 When 130 g of the crudecomplex product obtained Ethyl naphthalenes 2.3 0.0 0.0 1n Example I-lwere washed with 130 ml of methanol at other room temperature, 81.2 g ofa complex product comhydroca bons 2.8 0.7 0.0

prising 80.6 percent of the 2,6-DMN complex and 12.3 percent of the2,7-DMN complex were obtained. This complex product was recrystallizedfrom 105 ml of methanol to obtain 51.3 g of a purified complex productcomprising 95 .4 percent of the 2,6-DMN complex and 4.6 percent of the2,7-DMN complex.

EXAMPLE lI-2 One hundred and fourty grams of the crude complex. productobtained in Example [-2 were washed twice with 135 ml of methanol toyield 80.4 g of a purified complex product comprising 85.0 percent ofthe 2,6- DMN complex and 10.5 percent of the 2,7-DMN complex.

EXAMPLE Il-3 One hundred and twenty grams of the crude complex productobtained in Example [-2 were washed twice with 540 ml of n-heptane toyield 100 g of complex crystals comprising 83.2 percent of the 2,6-DMNcomplex and 12.3 percent of the 2.7-DMN complex.

EXAMPLE II-4 With use of 13 cc of toluene, 3.50 g of a complex mixtureobtained in the same manner as in Example [-5 were recrystallized. Theyield was 2.62 g. In order to determine the content of the 2,6-DMNcomplex content in the resulting complex product, a part of the productwas taken and dissolved in ether and decomposed by addition of a 5percent aqueous solution of sodium hydroxide. The ether layer wasanalyzed by the EXAMPLE lI-S With use of cc ofmethanol, 35.0 g of thesame complex product as used in Example II-l, the yield of therecrystallized product was 13.8 g. The results of the analysis of therecrystallized product conducted in the same manner as' in Example [14are shown in Table II- 2. 7

TABLE Il-2 Composition of recrystallized complex product by weight)2,6-DMN 78.2

2,7-DMN 15.4

Other DMN's 3.6

Ethyl naphthalenes 0.8

Other hydrocarbons 2.0

EXAMPLES 11-6 to 11-14 TABLE II-3 Solvent Hydrocarbon composition incomplex (percent by weight) Yield Ethyl Other Amount of com- )thm'naphhydro- Kind used (1111.) plex (,rr.) 2,6-DMN BE-DMN DMN's thalcncscarbons Example number:

ll-ti Bonznnonuu... 13 20 510.0 0.8 (1.2 0.1 1.13 ll 'l0lut\11t'. l3 42.20 03.5 5. 1 0.8 0. 3 0.0 H44 Xyluluun. 10 1.20 113.8 4.5 1.3 0.4 0.01l--11 MvSiLylunu 15 2. 35 113. 3 11. .1 11. 3 11.11 lll'Il.l1yl1-.l.l1(1'. .20 1.35 01.] 5. .1 1.8 0.5 0. T 1111 'lvtrahydrol'unur 111 0. 1111|). 7 4. 3 2. 4 0. 3 11.4 lll2 Dinxmw h 0.0-"r 02.0 [L8 0.0 0.0 0.0 lll3 1\l1-l.l1 vl1l.|1 \"lkt1.0111 N l. '10 02. 3 I. 5 l. X 0. -l 1.0 l Il4 Ethyl llttl tlll. 10 I. -10 01.0 7. l 0. 1'1 0. 5 0. 5

EXAMPLE 111-1 A vessel equipped with a reflux cooler was charged with51.3 g of the purified complex product obtained in Example II-l, and1500 ml of water were added thereto. Then the mixture was heated toreflux water. The DMN distilled by steam distillation was trapped toobtain 16.1 g of 2,6-DMN of a purity of 98.5 percent. The yield of2,6-DMN based on 2,6-DMN contained in the starting material was 67.6percent. After completion of the decomposition, the water layer wascooled to recover 33.5 g of m-nitrobenzoic acid.

EXAMPLE III-2 i In the same manner as in Example 111-1, 80 g of thepurified complex product obtained in Example II-2 were treated togetherwith 1350 ml of water. Thus, 22.8 g of a DMN mixture comprising 85.3percent of 2,6-DMN were obtained and 10.8 percent of 2,7-DMN. The waterlayer was cooled to recover 53.0 g of mnitrobenzoic acid.

The resulting DMN mixture (22.8 g) was recrystallized from 320 g ofmethanol to yield 16.1 g of 2,6-

DMN of a purity of 99.5 percent. The overall catching ratio of 2,6-DMNbased on the starting material was 66.6 percent.

EXAMPLE III-3 In ether 100 g of crystals obtained in Example II-2, weredissolved and 15 percent aqueous ammonia was added thereto.'The mixturewas shaked to decompose the complex. The ether layer was washed withwater and ether was distilled off to yield 20.2 g of a DMN mixturecomprising 83.6 percent of 2,6-DMN and 12.5 percent of 2,7-DMN. This DMNmixture was recrystallized from 49.8 g of n-heptane to yield 13.7 g of2,6- DMN of a purity of 96.7 percent. Distillation of n-heptane from themother liquor after the recrystallization gave 6.5 g of a DMN mixturecomprising 56.5 percent of 2,6-DMN and 31.8 percent of 2,7-DMN. When theaqueous ammonia layer obtained at the decomposition of the complex wasmade acidic with dilute sulfuric acid, 79.2 g of m-nitrobenzoic acidwere recovered.

EXAMPLE III-4 In 200 g of methanol 50.00 g of a crude complex productobtained according to the method of Example 1-3, were dissolved at roomtemperature and the solution was cooled to -15C. The precipitated solidsof a faintly yellowish white color were separated by filtration, andwashed with 50 g of methanol cooled to 0C. to yield 8.86 g of whitesolids (crude 2,6-DMN) comprising 83.5 percent of 2,6-DMN and 2.7percent of 2,7-DMN. The solids were recrystallized from 90 g of methanolto yield 5.48 g of 2,6-DMN ofa purity of 96.4 percent. The yield of2,6-DMN based on the starting material containing 12 percent of 2,6-DMNwas 61.7 percent. As a result of the alkali titration analysis it wasfound that this 2,6-DMN product comprised 0.68 percent by weight ofm-nitrobenzoic acid.

EXAMPLE III-5 precipitated solids were collected by filtration andwashed with 50 g of methanol cooled to 0C. to yield 4.31 g of crude2,6-DMN comprising 95.7 percent of 2,6-DMN and 3.9 percent of 2,7-DMN.The remaining filtrate and washing liquor were combined and cooled to15C. to precipitate solids of a faintly yellow color. The solids werecollected by filtration and washed with 50 g of methanol cooled to 0C.to yield crude 2,6- DMN comprising 86.5 percent of 2,6-D MN and 11.5percent of 2,7-DMN. Similar treatment of the filtrate and washing liquorwas repeated twice by changing the cooling temperature to 25C. and 27C.to obtain 1.84 g of crude 2,6-DMN comprising 80.5 percent of 2,6-DMN and19.6 percent of 2,6-DMN and 1.04 g of a crude DMN product comprising32.1 percent of 2,6- DMN and 53.6 percent of 2,7-DMN, respectively.These crude products were combined with crude products obtained above,and they were recrystallized. from g of methanol to obtain 7.23 g of2,6-DMN of a purity of 95.0 percent. The yield of 2,6-DMN based on2,6-DMN contained in the starting material was 80.0 percent. As a resultof the alkali titration analysis, it was found that the productcontained 0.87 percent by weight of m-nitrobenzoic acid.

COMPARATIVE EXAMPLE One hundred and ninety grams of the same DMNcontaining hydrocarbon mixture as used in Example [-2 were cooled to0C., and the precipitated white solids were separated by filtration toyield 16.0 g of a crude DMN product containing 45.01 percent of 2,6-DMNand 15.01 percent of 2,7-DMN. This crude product was recrystallized from50 ml of methanol and then from 75 ml of methanol to obtain 4.35 g of2,6-DMN of a purity of 99.9 percent. The yield of 2,6DMN based on2,6-DMN contained in the starting material mixture was 21.2 percent.

EXAMPLE III-6 A glass autoclave was charged with 20.0 g of the crudecomplex product obtained in Example l-l g of water and 10.0 g ofheptane, and the mixture was heated at C. for 20 minutes underagitation. At that time, the gauge pressure reached 2.0 kg/cm. Theagitation was stopped and 10 minutes later the water layer was separatedin that state. Distillation of the solvent from the organic layer gave7.23 g of a crude DMN product which contained 10.9 percent ofmnitrobenzoic acid. Together with 30 g of heptane and 110 g of water,the crude product was treated in the same manner as above to yield 6.50of DMN in which the content of m-nitrobenzoic acid was reduced to 1.2percent. From the water layer 12.2 g of m-nitrobenzoic acid wererecovered.

Example I1I-7 Together with 200 g of water and 6.8 g of heptane, 19.0 gof a complex product obtained by the same method as adopted in ExampleI-5 were treated at C. for 20 minutes in the same manner as in ExampleI1lg6 to yield 6.51 g of a crude DMN product which contained 6.3 percentof m-nitrobenzoic acid. This crude DMN product (6.51 g) was dissolved in68 g of heptane and contacted with 60 g of water at 60C. to

yield 6.13 g of DMN in which the content of mnitrobenzoic acid wasreduced to 0.62 percent. When the water layer was cooled, 11.9 g ofm-nitrobenzoic acid were precipitated and recovered.

EXAMPLE in-s EXAMPLE Ill-9 20.0 Grams of the complex product obtained inExample 1-1 were treated at 120C. for minutes with g of cyclohexane and200.0 g of water in the same manner as in Example 111-6, and the waterlayer was removed. Without separation of the crude DMN from the organiclayer, 500 g of water were added to the remaining organic layer and themixture was treated at 120C. for 15 minutes, followed by removal of thewater layer. From the remaining organic layer 6.48 g of DMN whichcontained 0.8 percent of m-nitrobenzoic acid were recovered.

EXAMPLE 111-10 10.00 Grams of the complex product obtained in ExampleI-l were treated at 75C. with 20 g of cyclohexane and 210 g of water inthe same manner as in Example 111-6, and the organic layer was washedtwice with 400 g of water heated at 75C. to yield 3.12 g of DMN whichcontained 1.5 percent of m-nitrobenzoic acid.

EXAMPLE Ill-11 16.0 grams of a complex product obtained according to thesame method as adopted in Example [-1 were treated at 140C. with 15.0 gof ligroin and 13.0 g of water in the same manner as in Example Ill-6 toyield 6.60 g of a crude DMN product containing 17.9 percent ofm-nitrobenzoic acid. Then, the crude product was treated at 150C. with32 g of ligroin and 110 g of water in the same manner as in Example111-6 to yield 4.60 g of DMN of a m-nitrobenzoic acid content of 3.2percent. When the water layer were cooled, 9.2 g of mnitrobenzoic acidwas precipited and recovered.

EXAMPLE Ill-12 20.0 Grams of a complex product obtained by the samemethod as adopted in Example 1-1 were vigorously agitated twice with 200g of water heated at 90C. and thrice with 100 g of water heated at 90C.When the water layer was removed, 5.50 g of a white solid product (DMN)were obtained. Cooling of the water layer gave 11.2 g of m-nitrobenzoicacid. As a result of the alkali titration analysis, it was found thatthe m-nitrobenzoic acid content of the resulting DMN product was 1.10percent by weight.

EXAMPLE 111-1 3 20.0 Grams of a complex product obtained according tothe same method as adopted in Example [-1 were contacted in an autoclavewith 250 g of water maintained at 120C., and the water layer wasseparated from the organic layer.

The above operation was repeated 5 times. Thus, 12.0 g of m-nitrobenzoicacid were recovered from the water layer, and 5.80 g of DMN whichcontained 0.7 percent by weight of m-nitrobenzoic acid were obtained.

EXAMPLE III-14 To 2.2 g of the complex product obtained in Example 11-5,20 cc of methanol were added and steam heated at 120C. was passedthrough the mixture for 15 minutes, whereby 0.4 g of DMN was distilled.off together with methanol and steam. The amount recovered ofm-nitrobenzoic acid recovered was 1.5 g.

EXAMPLE 111-1 5 To 2.2 g of the complex product obtained in Example1l-5, 20 cc of benzene were added and steam heated at 120C. was passedthrough the mixture for 15 minutes, whereby 0.65 g of DMN was distilledoff together with benzene and steam. The amount of mnitrobenzoic acidrecovered was 1.2 g.

EXAMPLE III-16 To 5.0 g of the complex product obtained in Example Il-5,200 cc of water were added and air heated at C. was passed through themixture for 1 hour while the mixture was being heated. Thus, 1.5 g of aDMN mixture were obtained.

EXAMPLE IlI-l7 and 0.8 g of 2,6-DMN and 0.1 g of faintly yellowishlycolored DMN comprising 5 percent m-nitrobenzoic acid were obtained. 7

What we claim is:

1. A process for separating dimethyl naphthalenes having a componentcomprising -2,6-dimethyl naphthalene in the form of complexes withmnitrobenzoic acid, which comprises contacting a dimethyl naphthaleneisomer mixture comprising at least 2,6-dimethyl naphthalene or ahydrocarbon mixture containing said dimethyl naphthalene isomermixturewith m-nitrobenzoic acid, to thereby form a mixture of complexes of thedimethyl naphthalenes with mnitrobenzoic acid having a main componentcomprising a complex of 2,6-dimethyl naphthalene with mnitrobenzoicacid, and separating the complexes in the solid state from the reactionmixture.

2. The process of claim 1, wherein m-nitrobenzoic acid is used in anamount of 0.1 5 moles per mole of 2,6-dimethyl naphthalene contained inthe starting dimethyl naphthalene isomer mixture or the startinghydrocarbon mixture containing said dimethyl naphthalene isomer mixturebut less than 2 moles per mole of the entire dimethyl naphthalenescontained in the starting mixture. 1

3. The process of claim 1, wherein the dimethyl naphthalene isomermixture comprising at least 2,6-

' mixture with m-nitrobenzoic acid is conducted in the presence of amonocyclic aromatic compound of six to nine carbon atoms.

6. The process of claim 2, wherein the dimethyl naphthalene isomermixture comprising at least 2,6- dimethyl naphthalene or the hydrocarbonmixture containing said dimethyl naphthalene isomer mixture is contactedin the liquid state with m-nitrobenzoic acid.

7. The process of claim 2, wherein the contact of the dimethylnaphthalene isomer mixture comprising at least 2,6-dimethyl naphthaleneor the hydrocarbon mixture containing said dimethyl naphthalene isomermixture with m-nitrobenzoic acid is conducted while both are kept in theliquid state.

8. The process of claim 2, wherein the contact of the dimethylnaphthalene isomer mixture comprising at least 2,6-dimethyl naphthaleneor the hydrocarbon mixture containing said dimethyl naphthalene isomermixture with m-nitrobenzoic acid is conducted in the presence of amonocyclic aromatic compound of six to nine carbon atoms.

9. The process of claim 3, wherein the contact of the dimethylnaphthalene isomer mixture comprising at least 2,6-dimethyl naphthaleneor the hydrocarbon mixture containing said dimethyl naphthalene isomermixture with m-nitrobenzoic acid is conducted in the presence of amonocyclic aromatic compound of six to nine carbon atoms.

10. The process of claim 4, wherein the contact of the dimethylnaphthalene isomer mixture comprising at least 2,6-dimethyl naphthaleneor the hydrocarbon mixture containing said dimethyl naphthaleneisomermixture with m-nitrobenzoic acid is conducted in the presence of amonocyclic aromatic compound of six to nine carbon atoms.

2. The process of claim 1, wherein m-nitrobenzoic acid is used in anamount of 0.1 - 5 moles per mole of 2,6-dimethyl naphthalene containedin the starting dimethyl naphthalene isomer mixture or the startinghydrocarbon mixture containing said dimethyl naphthalene isomer mixturebut less than 2 moles per mole of the entire dimethyl naphthalenescontained in the starting mixture.
 3. The process of claim 1, whereinthe dimethyl naphthalene isomer mixture comprising at least 2,6-dimethylnaphthalene or the hydrocarbon mixture containing said dimethylnaphthalene isomer mixture is contacted in the liquid state withm-nitrobenzoic acid.
 4. The process of claim 1, wherein the contact ofthe dimethyl naphthalene isomer mixture comprising at least 2,6-dimethylnaphthalene or the hydrocarbon mixture containing said dimethylnaphthalene isomer mixture with m-nitrobenzoic acid is conducted whileboth are kept in the liquid state.
 5. The process of claim 1, whereinthe contact of the dimethyl naphthalene isomer mixture comprising atleast 2,6-dimethyl naphthalene or the hydrocarbon mixture containingsaid dimethyl naphthalene isomer mixture with m-nitrobenzoic acid isconducted in the presence of a monocyclic aromatic compound of six tonine carbon atoms.
 6. The process of claim 2, wherein the dimethylnaphthalene isomer mixture comprising at least 2,6-dimethyl naphthaleneor the hydrocarbon mixture containing said dimethyl naphthalene isomermixture is contacted in the liquid state with m-nitrobenzoic acid. 7.The process of claim 2, wherein the contact of the dimethyl naphthaleneisomer mixture comprising at least 2,6-dimethyl naphthalene or thehydrocarbon mixture containing said dimethyl naphthalene isomer mixturewith m-nitrobenzoic acid is conducted while both are kept in the liquidstate.
 8. The process of claim 2, wherein the contact of the dimethylnaphthalene isomer mixture comprising at least 2,6-dimethyl naphthaleneor the hydrocarbon mixture containing said dimethyl naphthalene isomermixture with m-nitrobenzoic acid is conducted in the presence of amonocyclic aromatic compound of six to nine carbon atoms.
 9. The processof claim 3, wherein the contact of the dimethyl naphthalene isomermixture comprising at least 2,6-dimethyl naphthalene or the hydrocarbonmixture containing said dimethyl naphthalene isomer mixture withm-nitrobenzoic acid is conducted in the presence of a monocyclicaromatic compound of six to nine carbon atoms.
 10. The process of claim4, wherein the contact of the dimethyl naphthalene isomer mixturecomprising at least 2,6-dimethyl naphthalene or the hydrocarbon mixturecontaining said dimethyl naphthalene isomer mixture with m-nitrobenzoicacid is conducted in the presence of a monocyclic aromatic compound ofsix to nine carbon atoms.